The present invention relates generally to imagers having a focal plane array of infrared detectors and charge coupled device signal processing circuitry associated with the detectors to transfer the signals produced by the detectors into data processing apparatus. In this apparatus the image may be reconstructed from the signals in a manner determined by the particular mode of scanning employed. Imagers of this general type are disclosed, for example, in the following U.S. Pat. Nos.
B. K. Weimer, 3,683,193;
Y. Takamura, 3,777,061;
D. R. Collins, 3,771,149;
K. Nummedal, 3,883,437.
In the prior art, of which the foregoing patents are typical, efforts to integrate the photodetectors of such an array with a charge coupled circuitry have in the past led either to hybrid devices wherein the detectors were mounted on top of a separate semiconductor substrate in which the charge coupled circuitry was contained or to an arrangement wherein the photodetectors were formed as portions of the same semiconductor substrate containing the charge coupled circuitry.
The device disclosed and claimed in copending application Ser. No. 614,277, represents a successful monolithic integration of the detector and its associated CCD circuitry into one efficient and low-cost device. In the fabrication of this earlier structure disclosed in Ser. No. 614,277, an epitaxial layer is formed on an IR detector substrate and then a portion of that epitaxial layer is etched away as to permit infrared radiation to impinge on the individual detectors of the substrate. This produces charges which are then read out by direct injection into the charge coupled device which is built on the epitaxial layer and functions to transfer charges through the epitaxial layer and into the output of the device.
The device disclosed and claimed in copending application Ser. No. 702,548, eliminates the need for having a portion of the epitaxial layer etched away. It instead uses different means for collecting charges generated in the detector elements and injecting them into the CCD circuitry. Among the charge collection structures disclosed was the use of a conducting buried layer and conducting surface layer combination for the collection and transfer of charges from the detector elements to the CCD circuitry.
The device disclosed and claimed in copending application Ser. No. 720,865 comprises two epitaxial layers grown on a surface of an extrinsically doped silicon substrate. The CCD readout circuit is built in a portion of the second epitaxial layer. Between portions of the two epitaxial layers and just below the CCD circuit there is a degeneratively doped shield layer to isolate the underlying detector region from the CCD operation. Infrared radiation can reach the detector region by entering either through the epitaxial layers or by entering through a detector electrode in the opposite end of the detector. The generated carriers are collected and transferred through the use of a collection structure involving a first degeneratively doped buried layer in a portion of the interface between the first epitaxial layer and the detector region and a third degeneratively doped buried layer that connects the first buried layer with a degeneratively doped first surface layer. This device has an improved fill factor and improved signal-to-noise ratio over its prior art counterparts.
While the inventions by R. M. Finnila et al. disclosed and claimed in the aforementioned patent applications represent significant advances in this art as explained in detail in said applications, my present invention further and substantially extends the developments and advances in this area of technology. The present invention has the further advantages of improving the flexibility of the device for applications requiring periodic adjustments of resolution versus sensitivity. It provides for a device that can give a variable resolution and which can be adjusted to trade off resolution at the expense of sensitivity or vice versa.